(i) Field of the Invention
The invention comes within the general scope of the reduction of the content of greenhouse gases in gaseous effluents of industrial origin discharged to the atmosphere. It is a question here of lowering nitrous oxide N2O in gaseous discharges.
(ii) Description of Related Art
For a long time, concern was only felt about the discharge of nitric oxides (NOx), which easily combine with water to form nitrous or nitric acids, the most spectacular sign of which is without doubt acid rain, with subsequent destruction of forests and damage to exposed monuments, and the most insidious signs of which are contamination of breathable air and its effect on public health. Awareness has now arisen of the significant contribution of nitrous oxide to enhancing the greenhouse effect, with the risk of leading to climatic changes with uncontrolled effects, and perhaps also of its participation in the destruction of the ozone layer. Its removal has thus become a preoccupation of the authorities and of manufacturers.
While the most significant sources of N2O are the oceans, uncultivated soils, agriculture, the combustion of organic matter and the use of fossil fuels, the chemical industry contributes some 5 to 10% of emissions of this gas. Nitric acid plants, as well as plants for organic synthesis employing nitric oxidation processes (production of adipic acid, of glyoxal, and the like), are the source of most discharges of N2O by the chemical industry (see, in this respect, Freek Kapteijn et al., Heterogenous Catalytic Decomposition of Nitrous Oxide, in Applied Catalysis B, Environmental 9, 1996, 25-64).
For some years already, most nitric acid plants have been equipped with so-called DeNOx reactors, which operate satisfactorily in removing nitric oxides from their effluents. However, N2O, which is essentially produced during the oxidation of ammonia over the platinum gauzes of the burners, remains substantially constant between the outlet of the burners and the inlet of the DeNOx reactor and is not lowered by passage of the gases through this reactor (sometimes, it is even slightly increased).
Provision has been made to reduce the N2O content of the gaseous effluents resulting from nitric oxidation processes in organic chemistry by catalytically destroying the nitrous oxide contained in the latter over a mordenite/iron catalyst (EP 0,625,369). However, on account of the large fall in its activity in the presence of steam in the temperature range 350-450xc2x0 C., this catalyst is not well suited to functioning with respect to dilute gases and ages badly, due to a mediocre hydrothermal resistance.
It also turns out to be economically unsuited to the treatment of the tail gases from nitric acid plants, which, upstream of the expansion turbine, generally correspond to the following characteristics,
The economic optimization of the lowering of N2O both in the gases emitted by organic plants and by nitric acid plants involves the development of a catalyst which retains a good activity for the destruction of N2O at a temperature below 400xc2x0 C. in the presence of NOx and of steam, and which has a sufficient hydrothermal stability at 600xc2x0 C to withstand the temperature peaks to which it may be subjected under certain circumstances in its use.
A solution corresponding to such specifications has just been found with a catalyst composed of agglomerates formed of 80 to 90% of a ferrierite/iron assaying from 1 to 6% of iron, and preferably from 2 to 4%, and of 20 to 10% by weight of an agglomeration binder (percentages by weight with respect to the weight of the granule).
The ferrierite/iron is the active component of the catalyst according to the invention. The structure of its crystal lattice is that of ferrierite [RN=12173-30-7], that is to say a zeolite traversed by two systems of channels, one parallel to the c axis of the structure, formed of channels with an elliptical cross-section (0.43 nmxc3x970.55 nm) of approximately 0.18 nm2 (18 xc3x852), the other parallel to the b axis and the c axis of the structure, with channels formed of 8-membered rings, with 0.34xc3x970.48 nm axes. There is no channel parallel to the a axis. Approximately spherical cavities, with an approximate diameter of 0.7 nm, lie on these channels and are accessible only through the 8-membered rings, i.e. via 0.43 nmxc3x970.55 nm or 0.34 nmxc3x970.48 nm pores. The ferrieritic structure is completely characterized by its X-ray diffraction diagram (for the interlattice distances, consult Breck xe2x80x9cThe Synthetic Zeolitesxe2x80x9d, 1974 Edition, Table 4.45, p. 358).
This ferrierite/iron is obtained by subjecting a commercial ferrierite, of sodium/potassium type, to exchange with an aqueous solution of an iron salt, so as to obtain the desired iron content. The operating procedures are well known to a person skilled in the art. It is possible, in particular, to carry out one or more exchanges by immersion in an iron salt solution or by column percolation, either of the ferrierite powder itself or with respect to granules.
This exchange can be carried out either using a ferric salt solution or using a ferrous salt solution. Use is advantageously made of ferrous sulphate, which is a very low cost product and which does not introduce chlorides, which are sources of corrosion, into the preparation.
Preference is given to the form exchanged with iron starting from the ammonium form of ferrierite, which is obtained by subjecting a commercial ferrierite, the electrical neutrality of the crystallographic lattice of which is essentially produced by sodium and potassium alkali metal ions, to an exchange with a solution of an ammonium salt. The ferrierite/iron obtained from the ammonium form of ferrierite exhibits, as characteristic, that of having a very low content of alkali metal ions in the exchange position. It is the low content of potassium ions (less than 0.5% by weight) which analytically indicates this preferred form of the catalyst of the invention. The ferrierites/iron according to the invention contain only 0.5 to 0.1% of potassium.
The catalysts according to the invention are shaped as agglomerates, a presentation which is necessary for reasons of minimization of the pressure drop as they pass through the catalyst bed. The agglomeration of zeolites is well known to a person skilled in the art. It is carried out by forming a paste of the zeolite powder with a binder, generally fluidified with water, often composed of a clay which is simultaneously sufficiently plastic to be able to form the agglomerate as balls, using a dish granulator, as pellets by moulding or as extrudates, using an extruder, and hardenable by calcination to give sufficient cohesion and hardness to the agglomerate. The clays used are kaolinites, attapulgites, bentonites, halloysite or mixtures of these clays.
It is also possible to use siliceous or aluminous binders. In particular, agglomeration with peptized aluminas gives very strong granules, this method of agglomeration being possible here because ferrierite is not degraded by the acidity of the binder.
After agglomeration, the granules are thermally activated. This means that they are subjected to a calcination carried out under air at a temperature of approximately 400xc2x0 C., the role of which is both to harden the binder, to dehydrate it without hydrothermally degrading it and, in the case of ferrierites exchanged starting from an ammonium form, to remove a large part of the ammonium ions and to bring the zeolite to the H form.
It is also possible to start by agglomerating the sodium/potassium ferrierite, then to harden it by calcination and to carry out exchanges on the agglomerate. After drying, a second calcination makes it possible to bring the ferrierite/iron to the H form, if the ferrierite employed was taken in the ammonium form.
This catalyst is the improved catalytic means of a process for destroying N2O, contained in a gas mixture, according to the overall reaction:
2N2Oxe2x86x922N2+O2
This process, which is also one of the subjects of the present invention, consists in passing the gases to be purified, in which the range of concentrations of N2O extends from 500 ppm to 50%, of H2O from 0.5 to 5% and of NO from 50 to 2000 ppm, through a catalyst bed placed in an axial or radial flow reactor maintained at a temperature of between 350 and 600xc2x0 C. In the treatment of a gas with a high N2O content and with an initial temperature of less than 350xc2x0 C., as is generally the case in processes for organic synthesis by nitric oxidation, the initiation of the reaction can be facilitated by preheating, during the start-up phase, the gas flow or the catalyst by an external means, the temperature of the catalytic bed subsequently being self-supporting because of the exothermicity of the reaction. In certain situations, in particular in the case of the treatment of gas with a high N2O concentration, heat exchangers or devices of quench type can advantageously be immersed in the catalytic bed in order to control the temperature of the latter, it optionally being possible to use part of the heat to preheat the gas to be treated.
Contrary to other zeolitic catalysts, the ferrierite/iron according to the invention retains a manifest activity with respect to N2O in the presence of water. This activity is very much enhanced in the presence of NO, which is a very favourable factor because this synergy becomes more significant for very low levels of NO, of the order of 50 ppm, and because the gases capable of such a treatment almost always contain such traces of NO.
The process according to the invention finds its application in particular in the treatment of tail gases from nitric acid plants, both before and after DeNOx treatment, which gases can have compositions within the following limits,
the remainder being essentially composed of nitrogen.
The process can also be applied to the treatment of gases resulting from plants for organic oxidation using nitric acid in organic chemistry, in particular in the manufacture of adipic acid, of glyoxal and of glyoxylic acid. These are gases with the approximate composition, before optional dilution with air, as follows:
the remainder being essentially composed of nitrogen.